(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming metal contacts and Vias.
(2) Description of Prior Art
The use of tungsten in the fabricating of very-large-scale-integrated (VLSI) circuits has been practiced since the mid 1980s. As a conductive material tungsten does not rank as high as aluminum, which has been the primary conductor used in micro-circuit chip technology for nearly forty years. On the other hand, tungsten provides many features which make it an important material for fabricating metal-to-silicon contacts as well as via plugs for making inter-level metal connections. In this capacity tungsten is deposited into contact or via openings, and anisotropically etched to the insulating layer, leaving tungsten in the openings.
Chemical-vapor-deposited (CVD) tungsten has proven to be an excellent material for such interconnect applications because of its low resistance, low stress (less than 5.times.10.sup.9 dynes/cm.sup.2), and a coefficient of thermal expansion which closely matches that of silicon. In addition tungsten has a high resistance to electromigration which is a common problem with aluminum its alloys. CVD tungsten can be deposited at temperatures around 400.degree. C. with good conformity and step coverage.
Although tungsten does not bond well to either silicon or to the adjacent silica based insulating layer, a thin layer (less than 1,000 Angstroms) of titanium (Ti) is often used as a bonding agent to the silicon. Dixit et.al. U.S. Pat. No. 4,960,732 describe the formation of a tungsten plug contact utilizing Ti as a bonding agent, followed by a layer of titanium nitride (TiN) which acts as a diffusion barrier to prevent dopants from passing from the silicon as well as spiking of metal into the silicon. The Ti layer, when thermally annealed, fuses with the silicon to form titanium silicide (TiSi.sub.2) and with the silica based insulating layer to form a titanium silicate (Ti.sub.x SiO.sub.y). Adhesion of the TiN to the Ti and subsequently to the tungsten is considered excellent.
The Ti-TiN-W composite tungsten plug metallurgy has been widely accepted and various techniques for its formation have been described. In the earliest teachings such as those of Dixit et.al., the Ti an TiN layers were deposited by sputtering, although CVD is also claimed. The sputtering can be accomplished by first sputtering a titanium target with argon to form the Ti layer and then admitting nitrogen, thereby sputtering reactively, to form the TiN layer. Alternatively, a multi-target sputtering tool can be used having a Ti target and a TiN target so that the layers may be deposited during a single evacuation cycle by switching targets within the tool. Successive deposition of the Ti and the TiN layers during a single pumpdown is desirable because exposure of the Ti layer to atmosphere will immediately result in the formation of a native oxide layer which can compromise the resistivity of the contact if not removed prior to the deposition of the TiN.
The good conformity and step coverage afforded by tungsten is due in large part to the nature of the deposition process itself. In the CVD process, particularly with low-pressure-chemical-vapor-deposition (LPCVD), the chemical reaction which forms the product occurs at the heated surface of the material receiving the deposition. Physical-vapor-deposition (PVD) processes such as evaporation or sputtering, on the other hand, cannot provide such conformity and edge coverage because the material being deposited arrives from discrete regions distant from its final location. This lends directionality to the process and consequently those regions of a receiving substrate which face the source of the particle stream receive a greater amount of deposit than those topological features not normal to the particle stream.
A computer simulation of film deposition by dc magnetron sputtering into an opening similar to that used for integrated circuit contacts is shown in FIG. 1 (adapted from S. Wolf and R. N. Tauber, "Silicon Processing for the VLSI Era", Vol.I, Lattice Press, Long Beach, Calif., (1986) p.368). A layer of material 8 of nominal thickness t.sub.n is sputter deposited onto a substrate material 6 having an opening with base w and height h. The step coverage is defined by the minimum thickness of the film t.sub.s divided by its nominal thickness t.sub.n expressed in percent. In this example the opening has a taper expressed by the angle 90-.theta. which favors better step coverage than the vertical walled contact openings under consideration here.
The minimum thickness in the contact openings occurs near the base as is also shown in FIG. 2A. Openings with a high aspect ratio h.div.w (FIG. 1) can have insufficient coverage along the base corners of the opening. Step coverage can be improved to some extent by heating of the wafer. This allows surface migration of the depositing species to occur, thereby improving conformity.
Consequently the sputtering processes for the deposition of Ti and TiN have the shortcomings of poor step coverage. This is illustrated in a prior art cross section of a contact opening shown in FIG. 2A. A semiconductor wafer 10, having an active area diffusion 12 has an insulating layer 14 into which an opening 15 has been made and a thin layer of Ti 16 followed by a thicker layer of TiN 18 have been deposited by sputtering. Inadequate step coverage causes cusps 20 to form at the entrance of the opening 15. A subsequently deposited tungsten layer, which is postured to fill the opening 15, prematurely pinches off before the lower portion of the opening 15 is filled. The result is a void 26 shown in FIG. 2B. When the tungsten layer is etched back into the insulating layer 14, completing the formation of the tungsten plug contact, the void 26 may be exposed creating a potential reliability defect. Exposed voids are highly susceptible to absorption of moisture or other corrosive contaminants.
Chen U.S. Pat. No. 5,462,895 also points out this step coverage shortcoming but does not indicate the occurrence of voids in the tungsten. However, such voids have been observed by scanning-electron-microscopy (SEM). They have also been encountered by Cheffings et.al. U.S. Pat. No. 5,387,550 who, rather than seeking to avoid them, treats them after-the-fact by over-etching the tungsten plug into the void and filling the void and the top of the opening with polysilicon. In doing so they also etch deeply into the TiN along the walls of the contact openings.
Chen resolved the edge coverage problem by using CVD for the deposition of the Ti and TiN layers, thereby achieving better conformity. However, because of thermal budget, availability, and other processing restraints, CVD is not always a viable option.
Yamada U.S. Pat. No. 5,470,792 and Matsuura U.S. Pat. No. 5,420,070 disclose methods for the formation of Ti and TiN layers within contact and via openings where tungsten plugs are used. They do not, however, disclose the use of chemical-mechanical-polishing (CMP) in combination with spin-on-glass (SOG) to form improved contacts or vias as is disclosed by this invention.
For applications where aluminum metallization is applied into the openings, whether contacts or vias, similar problems are encountered. Referring again to FIG. 2A, coverage of TiN in the vicinity of the base corners 22 of the opening 15 can be precariously thin so as to place the barrier functionality of the layer at risk. When contacts are involved, inadequate barrier function is known to cause aluminum migration leading to junction spiking.